1. Field of the Invention
The present invention relates to the technical field of semiconductor electronics and the present invention particularly relates to the technical field of bulk acoustic wave resonators (BAW resonators).
2. Description of Related Art
Bulk acoustic wave resonators (BAW resonators) are often used in electronics as cheap and easily manufacturable resonator structures. An example of a BAW resonator is illustrated in greater detail in FIG. 4. Here, an active resonator region 1 having a top electrode 2 and a bottom electrode 3 between which a piezo-electric layer 6 is arranged is illustrated. The top electrode 2 and the bottom electrode 3 also serve as feeding or excitation electrodes to cause a mechanical oscillation in the piezo-electric layer 6 when operating the BAW resonator. Furthermore, the bottom electrode and a part of a main surface of the piezo-electric layer 6 contact an embedding layer 5 which is, for example, made of SiO2. Put differently, the active resonator region 1 is thus arranged on the embedding layer 5. Additionally, one or several mirror layers 4 are arranged below the active resonator region 1 in the embedding layer 5, as is, for example, shown in FIG. 4 using the two mirror layers 4. Furthermore, the embedding layer 5 is arranged on a substrate 7.
When the active resonator region 1 is excited by applying a high-frequency voltage signal between the top electrode 2 and the bottom electrode 3, i.e. when the piezo-electric layer 6 is caused to oscillate mechanically by the voltage applied between the electrodes 2 and 3, a standing wave 8 will form in the BAW resonator between the active resonator region 1 in combination with the underlying mirror layers 4 buried in the embedding layer 5.
It is to be mentioned with regard to the BAW resonators that laterally propagating leaky waves 9 also occur when forming a standing wave 8 which transport energy from a region where the standing wave forms (i.e. the resonator region 1 with the underlying mirror layers 4) and thus decrease the quality of the BAW resonator. This is exemplarily illustrated in the Smith diagram illustrated in FIG. 5 where such a conventional standard structure, illustrated in FIG. 4, of a BAW resonator is identified by means of a continuous line. The decrease in quality of conventional BAW resonators can be recognized by a smaller radius of the corresponding characteristic curve in this Smith diagram, the characteristic curve being characterized by a greater distance to the unit circle (short-dashed line) in particular in the range of high impedances. This becomes particularly evident in the enlarged portion 52 of the Smith diagram 50. It is to be pointed out in general that the quality of a resonator will be the higher the closer its characteristic is to the unit circle. A considerable difference between a conventional standard structure and the unit circle can be recognized in particular in the area of a parallel resonance (high impedance, point at (1,0)).
In the theory of BAW resonators, it is assumed that an exponentially decreasing “evanescent” wave provides for a perfect lateral enclosure of the acoustic energy outside the electrode area. The evanescent wave condition is usually fulfilled in membrane BAW resonators since the surroundings of the resonator have a higher resonant frequency. In BAW resonators on acoustic mirrors, there is, however, a plurality of branches in a corresponding dispersion diagram and the enclosure of the acoustic energy will never be perfect. The loss of energy and the decreasing quality connected thereto have, up to now, been accepted.